In some cases, it is reported to work perfectly fine out of the box, but in several cases, especially with AMD cards, the game does not seem to recognize any resolution beyond a single monitor.
A specific game launcher, called RomTerraria 2.0 will allow you to launch the game at the desired resolution.

Check to make sure you have the most recent updates to your graphics card. You can do this through Steam (in Windows) by pressing (in the top right corner) 'Steam' > 'Check for Video Driver Updates...'.